DNA - Unit 2


Structure of DNA - Specification

From Specification

DNA is in the form of a double-helix

It is an information-carrying molecule

It's stable due to:

  • The components of nucleotides - deoxyribose (sugar), phosphate and the bases (adenine, cytosine, guanine and thymine)
  • Two sugar-phosphate backbones held together by hydrogen bonds between base pairs
  • Specific base pairing
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Structure of DNA


  • DNA is a polynucleotide. The sugar and phosphate is always the same but the base vaires
  • The organic bases in nucleotides pair specifically, a single ringed base pairs with a double ring base. The pairs are complementary to each other. Called specific base pairing
  • Cystosine pairs with Guanine with 3 hydrogen bonds
  • Thymine pairs with Adenine with 2 hydrogen bonds
  • Nucleotides combine together via condensation reactions, from a mononucleotide to a dinucleotide. A long chain is called a polynucleotide

DNA Structure

  • Nucleotides join to make sugar-phosphate backbone. Join between a phosphate and a sugar
  • Made of two polynucleotides joined together by hydrogen bonds
  • But only one polynucleotide contains the genetic code
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Linking Structure to Function

DNA contains the hereditary material which passes genetic information to each cell and generation. Contains genetic info to develop egg to adult

The infinite variety of base sequences provides variety within a species

It is built for its function by:

  • Double-helix makes it very stable and can be passed between generations without being damaged
  • Hydrogen bonds can easily separate in replication and protein synthesis
  • They are extremely long and coiled up tightly so lot of genetic info fits into small nucleous
  • Base pairs within the deoxyribose-phosphate backbone so are protected
  • The paired structure makes it easier to copy itself in self-replication. Important for cell division and passing infor from one generation to the next
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Genes and Polypeptides

Genes coding for proteins

  • A gene occupies a fixed position on a particular strand of DNA - the locus
  • Genes are sections of DNA made up of a specificsequence of baseswhich code for polypeptides. These determine the nature and development of organisms.
  • The base sequence of a gene can change as a result of a mutationproducing one or more alleles of the same gene.
  • A sequence of three bases, called a triplet, codes for a specific amino acid.
  • The base sequence of a gene determines the amino acid sequence in a polypeptide (primary structure). This means it determines the proteins made, determining your development

Introns - sections of DNA which dont code for amino acids. Include the repeats. Removed during protein synthesis
Exons - sections of DNA which code for amino acids

  • In eukaryotes, much of the nuclear DNA does not code for polypeptides
  • Mutations can produce new alleles of genes. This will code for a different amino acid so can mean the production of a non-functional enzyme and prevent important reactions happening
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The Triplet Code


Sections of DNA which code for a specific protein (polypeptides)

Genes therefore determine which enzymes are made. Enzymes are involved in the chemical reactions which determine the nature and development of an organism

The sequence of bases determines sequence of amino acids which determines shape and function of the protein made which determines the characteristics of a cell

Locus - the precise location of a gene on a chromosome

Genes also contain non-coding DNA called intron

Triplet code

The idea that 3 bases code for 1 amino acid

There are more amino acids than bases, so they must combine to code for an amino acid

An amino acid can be coded for by more than 1 code. This makes them degenerate

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DNA and Chromosomes


  • In eukaryotes, DNA is linear, exsist as chromosomes and are associated with proteins.
  • The proteins offer support to the DNA
  • In prokaryotes, DNA molecules are shorter, circular and are not associated with proteins
  • Instead of condensing by associating with proteins, the DNA fits via supercoiling


  • Only visible when the cell is dividing and appear as 2 threads (chromatids) joined at one point (the centromere)
  • The DNA is held in place by combining with proteins
  • The DNA-protein complex coils and these fold to form loops. The loops coil and pack together to form a chromosome
  • The number of chromosomes is the same within a species but varies between species
  • Members of the same species have the same genes with different alleles

Alleles - Different version of the same gene

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Homologous Chromosomes

Pairs of chromosomes which are the same shape and size and carry the same genes. One has come from the mother (maternal) and the other from the father (paternal)

Chromosomes occur in homologous pairs so there's always an even number of chromosomes

The total number of pairs is called the diploid number. In humans, this is 23.

Both chromosomes contain the same genes but different alleles. This means they determine the same characterisitc but are not identical. Eg. Colour of eyes

Each allele codes for a different polypeptide

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